The maintenance of mitochondrial function and integrity are crucial for normal cell physiology, particularly for cells with high energy demand. Neurons, for example, rely on the ATP generated by mitochondria through oxidative phosphorylation (OXPHOS) for functions such as synaptic transmission and channel activity. In addition to OXPHOS, mitochondria mediate cell death and survival by integrating cellular stress signals, and are the main source of reactive oxygen species (ROS), a cause of oxidative stress. Consequently, mutations in mitochondrial DNA (mtDNA) and nuclear DNA that affect mitochondrial respiration, morphology, and mtDNA content have been associated with a wide range of neuromuscular and neurological disorders including Down syndrome (DS). In DS cultures and tissues, there are reports of altered mitochondrial enzyme activities, elevated ROS, and defective mtDNA repair after oxidative damage, but the underlying cause of defective mitochondrial respiration is still unclear. The Down syndrome critical region 1 (DSCR1) gene, located in the region 21q22.1-q22.2 of chromosome 21, is overexpressed in DS fetal brain. DSCR1 is highly conserved across species and belongs to a family of proteins called calcipressins, which bind and inhibit calcineurin. Using Drosophila as a model system, we previously demonstrated that the Drosophila homolog of DSCR1, nebula, is required for learning and long-term memory through its regulation of calcineurin-mediated signaling pathway. We have shown that both nebula loss-of-function and overexpression flies have defective learning, suggesting that a delicate balance in the level of nebula is crucial for normal neuronal functions such as learning. In addition, studies done in cell culture suggest that DSCR1 may have functions involving regulation of oxidative stress response. Given the importance of mitochondria in oxidative stress response and neurological disorders, we hypothesized that DSCR1 may mediate mitochondria-dependent processes. Using the previously established Drosophila model system, we show that the correct level of nebula is crucial for normal mitochondrial respiration, level of ROS generation, maintenance of mtDNA content, and mitochondria number and size. Interesting, nebula is located in the mitochondria and interacts with a mitochondrial protein ? the ADP/ATP translocator (ANT) ? and influences its activity. Consistent with our findings, trisomy 21 fetal brain tissues with DSCR1 overexpression also display reduced mtDNA content. Taken together, these results indicate that nebula/DSCR1 is an important player in the maintenance of mitochondrial function and integrity.